1. Field of the Invention
The present invention relates to optical storage media and more particularly to servo tracks incorporated into the structure of a multilayer optical storage medium adapted for vesicular storage of data written by optical techniques.
2. Description of Related Art
U.S. Pat. No. 4,285,056 dated Aug. 18, 1981 of Bell for "Replicable Optical Recording Medium" shows a multilayer stack of Al, MgF.sub.2 and Ti as reflective, transmissive and reflective layers respectively. These form markings when the MgF.sub.2 vaporizes under the laser- heated Ti layer. No servo marks are provided, or suggested by the reference, which would allow an optical beam to detect and to follow predetermined tracks on the medium before data is written upon it.
U.S. Pat. No. 4,318,112 dated Mar. 2, 1982, of Kivits et al for "Optical Recording Disc" describes an optical storage medium which shows holes indicating data formed by ablation of a thin optically absorbing film. The holes are located along depressed servo tracks. FIGS. 3-5 of that patent show a servo track 16, a recording layer 18, a hole 20 formed by a laser beam and an annular ridge portion 21 of recording material 18 originating from the hole 20. Thus, the servo data that allows an optical beam to follow an unrecorded track is formed by grooving the surface of the medium 18 with the ridges 16 forming the servo tracks.
By contrast, it is an object of the present invention to eliminate the grooves on the exterior surface of the medium, thereby providing a medium with a substantially smooth exterior surface.
U.S. Pat. No. 4,175,145 of Fechter dated Nov. 20, 1979 for "Making Memory Structure for Laser Recording System" shows the use of layer 46 (FIG. 5 of that patent) to eliminate the effects of variations 47 and 48 in the substrate. The depressions 47 and protrusions 48 are defects in the surface of the substrate which are smoothed out. This pertains to preparation for recording data on a smooth polymeric surface prior to heating the surface with a laser beam to provide deformations representing data. It is an object of the present invention to provide servo tracks, to a medium having such a smooth surface medium, without deforming any of the surface on which the data is recorded for the purpose of producing servo markings on the medium.
European Patent Application No. 0 033 430 Al published Aug. 12, 1981, for "A Thermal-Optical Process for Inscribing Information and an Information-Bearing Device Designed for the Practical Application of Said Process" shows vesicles formed by a metal film on an organic layer over a transparent substrate. In FIG. 9 of that application, the vesicles are coated with a thick film of a metal to provide long life, after data has been recorded in the form of vesicles.
W. Robbins, R. Freese, T. Smith, and R. Wilson presented paper No. ThL2, "Bubble Forming Media for Optical Recording: a New Approach" at CLEO 1981 in Washington D.C., June 10-12, 1981. It described a bubble forming material for optical storage applications. The basic structure consisted of a gas-evolving polymeric material onto which an absorbing metallic film is deposited. Data is written by heating the metallic film locally with a focussed laser beam. The high temperature produced by the heating causes gas to be evolved from the polymeric material. When the metal and the polymeric material are chosen correctly, the gas causes the metallic film to bubble, thus resulting in a permanent local change in optical properties. It is an object of this invention to provide servo data which is recorded without using any of the surface area to be utilized for forming bubbles.
A problem with the above material is the difficulty encountered in supplying prerecorded information such as sector and track numbers and clock data on the disk. Because of the nature of the writing mechanism employed, and because the absorbing film is quite thin, the surface is required to be planar to achieve a uniform continuous film that does not have localized weak spots that could be caused by the topology of the surface. This requirement precludes incorporation on the bubble forming or vesicular medium of pre-recorded information in the form of pits or grooves which deform the active layer. Such information can be used for tracking purposes as is commonly done with ablative storage media.